Steroid hormones are responsible for many observed sex differences in brain and behavior in a variety of species, including humans. The female Asian musk shrew (Suncus murinus) is an insectivore that differs from most other mammals in its 1) lack of behavioral estrus cycle and 2) continuous testosterone release by the ovaries, rather than estrogen or progesterone. We have found that aspects of musk shrew sexual differentiation more closely resemble the pattern found in primates than that in more commonly studied animals like rats and mice. We propose to examine sex differences and hormonal control of the development and display of several sexually dimorphic behaviors in this species, including copulatory behaviors and spatial learning. We will investigate the role of developmental hormones in sexual differentiation by treating females neonatally with testosterone and its major metabolites, then examining sexually dimorphic behaviors and related brain structures in adulthood. We hope to add to the body of basic research in a rarely-studied mammalian order while at the same time provide students in our small women's liberal arts college with a valuable research experience that will enhance their laboratory skills and provide training necessary for success in graduate programs and research-related careers. (RELEVANCE TO PUBLIC HEALTH) In the regulation of sexual behavior, the musk shrew differs from commonly studied rodents but shares certain similarities with primates, including humans: 1) females mate at times other than a distinct estrus period;2) developmental testosterone does not require conversion to estrogen to shape male-typical sexual behavior;and 3) sexual interest develops prior to gonadal maturity, probably under the influence of adrenal androgens. Since basic research in primates is expensive, time-consuming and often otherwise impractical, we propose to develop an alternate small-animal model in a species that may more closely resemble humans than do rats and mice. We believe that an animal model for sexual differentiation will be useful to understanding how gonadal hormones shape the developing human brain and influence observable sex differences in later life.